This invention, broadly, relates to bearings. More particularly the invention pertains to hybrid bearings. In a still more specific aspect the invention relates to bearings for rocket engine turbopumps.
In rocket engines power is usually obtained by burning fuel and oxidizer which are mixed and directed to a combustion chamber. In these fuel feed systems turbopumps pressurize the propellants which are injected into the combustion chamber. The turbine must provide adequate shaft power for driving these propellant pumps. The turbopumps, then, are driven by the hot combustion products, generally combustion products of hydrogen and oxygen.
Roller bearings are generally used in turbopumps of rocket engines. However, because of bearing demands hydrostatic bearings have been proposed. The use of such bearings is quite feasible because during flight hydrostatic lubrication can reduce roller bearing wear. A disadvantage of such proposals is that during startup, acceleration and shutdown high pressure fluids are not available for hydrostatic bearings. Thus, unlike uses of normal hydrostatic bearings, the fluid lubrication film is not always present in bearings of turbopumps. During periods when the rotational speed of the Journal is too low, or when fluid pressure falls below the critical value for the hydrostatic bearing, the fluid film is ruptured and metal to metal contact increases friction and wear. A second bearing is, then, required to carry the load during such startup and shutdown periods. This requirement suggests the use of hybrid bearings in rocket engine turbopumps.
A characteristic of turbopumps is that they are high speed machines. They operate at shaft speeds as high as 93,000 rpm. Such high speed machines should utilize fluids as bearings to mitigate wear and tear. This criterion also suggests the use hybrid bearings.
A hybrid bearing for turbopumps is described in NASA Final Report CR-168124, RI/RD83-104 entitled Hybrid Hydrostatic/Ball Bearings in High-Speed Turbomachinery, January, 1983. In this report it is emphasized that vehicle requirements for future space maneuvering missions indicate the need for development of small, high-pressure liquid hydrogen turbopumps. These missions require high-speed operation for a long life, with many starts in a unit of minimum weight and envelope. Past efforts have included fluid dynamic and mechanical analysis, as well as design considerations to produce a liquid hydrogen turbopump for a 20,000 pound-thrust staged-staged combustion cycle engine for orbital transfer vehicle applications. The objective of the program reported in the Final Report was to retrofit a Mark 48 fuel turbopump with hybrid hydrostatic-ball bearings. This hybrid bearing is shown in FIG. 1. Referring to that figure, motor shaft 4 and, in this instance, duplex ball bearings 6 and 8, including balls 9 and 11, in inner races 10 and 12 and outer races 14 and 16 can be seen. The inner races, 10 and 12, are locked on shaft 4. The outer races, 14 and 16, in turn are locked in a ring or rotating annular sleeve or collar 18. This unit is contained within bearing housing, or shell, 20 adapted to provide a flow path 22 for the hydrostatic fluid, usually a portion of the turbopump outlet liquid oxygen. This liquid enters through channels or conduits 23 in housing 20, and flows both ways in flow path 22 between collar 18 and housing 20. This lifts both the bearing housing and the shaft or journal of the turbopump to the position shown in FIG. 1, thus supporting the journal when the pressure and quantity of flow are in correct proportions.
As shown in FIG. 1 rotating collar 18 terminates in a thrust-ring 24 which cooperates with an opposite thrust plate 26. Thrust plate 26 is carried by bearing housing 20 so that if pump thrust drives thrust ring collar 18 in that direction it will impinge on thrust plate 26 as a stop means.
Having described the hybrid bearing illustrated in FIG. 1, it is to be observed that even in the hydrostatic state duplex roller bearings 6 and 8 rotate with Journal 4. A disadvantage of that hybrid bearing is that at some increased speed, collar 18 will not be able to rotate as fast as journal 4. This is particularly true because the hydrostatic bearing pressure and flow supplied by the turbopump increase as pump speed increases. In addition the increasing pump speed creates hydrostatic bearing stiffness causing collar 18 to lag behind Journal 4. The result of this speed difference is that the roller bearing continues to wear. Another disadvantage is that duplex bearings do not wear evenly. These disadvantages are obviated herein by the provision of a hybrid hydrostatic-rolling element bearing wherein the rolling element bearing is locked to the stationary housing rather than on the rotating journal.